1. Field of the Invention
The present invention relates to a xerographic printing machine, and more particularly, to improvements of a transfer unit for a xerographic printing machine.
2. Description of the Related Art
In conventional xerographic printing machines, a transfer system based on corona discharge has been used for transferring a toner image formed on a photoreceptor to a print media. This type of transfer system is disadvantageous in that under high humidity conditions, image transfer performance is poor due to dust that accumulates on the corona wire and/or corrugations of the print media. To cope with this, a corona discharge transfer system in which the print media is pressed against a photoreceptor through a belt made of material of which the volume resistance is 10.sup.9 to 10.sup.14 .OMEGA.cm, has been proposed. In such a transfer system, the transfer performance varies largely due to a change of the electrical characteristics of the belt.
FIG. 4 is a sectional view showing the construction of a transfer unit for a conventional xerographic printing machine. FIG. 5 is a longitudinal sectional view of a charging unit used in the machine of FIG. 4. Endless belt 1 is stretched and wound, at a preset tensile force, around drive roller 2 and follower rollers 3, 4, and 5, so as to press paper 15 against photoreceptor 8. To secure a normal image transfer, the surface potential of the belt must be kept at a proper value. Ionic materials produced during the corona discharge by charging unit 14, dust in the air, toner, and the like, are attracted to, and deposited on, the surface of endless belt 1. As a result, the electrical resistance and dielectric constant of endless belt 1 change with time. This leads to a decrease in the surface electrical potential of the belt. As a result, the quantity of charge transferred to the print media changes, and hence the image transfer performance changes.
To cope with these limitations, conventional machines use suction means 9, provided near charging unit 14, to exhaust gases, such as O.sub.3 and Nox. The result is to create a negative pressure within transfer unit 7. With the negative pressure created, dust and toner, together with air, are introduced into the transfer unit, so as to soil the reverse side of endless belt 1. To prevent contamination by foreign materials, the conventional machine seals the gap between endless belt 1 and the frame. The sealing diminishes the gas vacuum action by suction means 9. Accordingly, endless belt 1 must still be replaced with a new one frequently.